Coating compositions

ABSTRACT

Silicon nitride coated crucibles for holding melted semiconductor material and for use in preparing multicrystalline silicon ingots by a directional solidification process; methods for coating crucibles; methods for preparing silicon ingots and wafers; compositions for coating crucibles and silicon ingots and wafers with a low oxygen content.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/837,873, filed Jul. 16, 2010, which claims the benefit ofU.S. Provisional Application No. 61/226,175, filed Jul. 16, 2009, U.S.Provisional Application No. 61/226,176, filed Jul. 16, 2009 and U.S.Provisional Application No. 61/226,172, filed Jul. 16, 2009, each ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

The field of the disclosure relates to coated crucibles for holdingmelted semiconductor material and, particularly, for use in preparingmulticrystalline silicon ingots by a directional solidification process.Other aspects include methods for coating crucibles, methods forpreparing silicon ingots and wafers, compositions for coating cruciblesand silicon ingots and wafers with a low oxygen content.

Conventional photovoltaic cells, used for the production of solarenergy, utilize multicrystalline silicon. Multicrystalline silicon isconventionally produced in a directional solidification (DS) process inwhich silicon is melted in a crucible and directionally solidified in aseparate or in the same crucible. The solidification of the ingot iscontrolled such that molten silicon solidifies unidirectionally at thesolidifying front of the casting. The multicrystalline silicon producedin such a manner is an agglomeration of crystal grains with theorientation of the grains being random relative to each other due to thehigh density of heterogeneous nucleation sites at the crucible wall.Once the multicrystalline ingot is formed, the ingot may be cut intoblocks and further cut into wafers. Multicrystalline silicon isgenerally the preferred silicon source for photovoltaic cells ratherthan single crystal silicon due to its lower cost resulting from higherthroughput rates, less labor-intensive operations and the reduced costof supplies as compared to typical single crystal silicon production.

During and after solidification, the solidified ingot must be releasedfrom the crucible without causing cracking of the ingot. Conventionalcrucibles are constructed of formed and sintered silica or fused-silica(synonymously “quartz”), silicon nitride or graphite. Silicon nitridecrucibles may be reused but are typically more expensive relative toother crucibles. Crucibles may be coated with various coating materials;however, these processes and the resulting coated crucibles have beenfound to be deficient.

SUMMARY

One aspect of the present disclosure is directed to a composition forcoating the inner surface of a crucible to improve an ingot-releasecharacteristic of the crucible. The composition includes a medium,silicon nitride, binder and a sintering agent.

In another aspect, a composition for coating the inner surface of acrucible to improve an ingot-release characteristic of the crucibleincludes a medium, silicon nitride, dispersant and a binder to enhanceadhesion of the coating to the crucible.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a crucible body according to oneembodiment of the present disclosure;

FIG. 2 is a perspective of a crucible according to one embodiment withheight, H₁ shown;

FIG. 3 is a perspective of a crucible body according to one embodimentwith the solidification line, S₁, and top, T, shown;

FIG. 4 is a perspective of a crucible according to one embodiment withheights, H₁ and H₂ shown; and

FIG. 5 is a graphical illustration comparing the oxygen concentrationsalong the height of a conventionally prepared crucible and a crucibleprepared by the process of one embodiment of the present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

Applicants have found that by coating a crucible such as, for example, asilica crucible, with a coating composition of sufficient thickness thatcontains minimal sources of oxygen, multicrystalline ingots and waferswith a reduced oxygen content and correspondingly reduced light induceddegradation may be produced. Further, oxygen-containing sintering aidsmay be used in various coating compositions applied to the crucible. Afirst area of the crucible body may be coated with a coating compositionthat does not contain the sintering aid and a second area coated with acomposition that does contain the sintering aid to minimize the surfacearea of the crucible exposed to oxygen.

By controlling the amount of oxygen in the coating compositions, theresulting ingots have been found to be characterized by a beneficialoxygen profile as compared to conventional processing methods andcoatings. It has been found that lower oxygen levels may be achieved inthe ingot which is desirable in, for example, boron-doped solar silicon.High oxygen levels in boron-doped solar silicon have been associatedwith light-induced degradation over time.

Crucible Body Starting Material

Referring now to FIG. 1, a crucible body for use in embodiments of thepresent disclosure is generally designated as numeral 5. The cruciblebody 5 has a bottom 10 and a sidewall 14 that extends from the base orbottom 10. While the crucible body 5 is illustrated with four sidewalls14 being shown, it should be understood that the crucible body 5 mayinclude fewer than four sidewalls or may include more than foursidewalls without departing from the scope of the present disclosure.Also, the corners 18 between sidewalls 14 may be connected to each otherat any angle suitable for forming the enclosure of the crucible body andmay be sharp as illustrated in FIG. 1 or may be rounded. In someembodiments, the crucible body has one sidewall that is generallycylindrical in shape. The sidewalls 14 of the crucible body 5 have aninner surface 12 and an outer surface 20. The crucible body 5 isgenerally open, i.e., the body may not include a top. It should benoted, however, the crucible body 5 may have a top (not shown) oppositethe bottom 10 without departing from the scope of the presentdisclosure.

In several embodiments of the present disclosure, the crucible body 5has four sidewalls 14 of substantially equal length (e.g., the cruciblehas a generally square base 10). The length of the sidewalls 14 may beat least about 25 cm, at least about 50 cm or even at least about 75 cm.The height of the sidewalls 14 may be at least about 15 cm, at leastabout 25 cm or even at least about 35 cm. In this regard, the volume ofthe crucible (in embodiments wherein a square or rectangular base isused or wherein the crucible is cylindrical or round or in embodimentswherein another shape is used) may be at least about 0.05 m³, at leastabout 0.15 m³ or at least about 0.25 m³). Further in this regard, itshould be understood that crucible shapes and dimensions other than asdescribed above may be used without departing from the scope of thepresent disclosure. In one or more particular embodiments of the presentdisclosure, the crucible body 5 has four sidewalls 14 that are eachabout 87.7 cm in length and 40 cm in height and the crucible has avolume of about 0.31 m³.

The crucible body 5 may be constructed of any material suitable for thesolidification of semiconductor material. For example, the crucible maybe constructed from a material selected from silica, silicon nitride,silicon carbide, graphite, mixtures thereof and composites thereof.Composites may include, for example, a base material with a coatingthereon. Composite materials include, for example, silica coated withsilicon nitride and graphite coated with calcium chloride and/or siliconnitride. It should be noted that some crucible body materials may notinherently be a source of oxygen contamination (e.g., graphite), howeverthey may have other attributes to be taken into consideration whendesigning a system (e.g., cost, contamination and the like). Inaddition, the material preferably is capable of withstandingtemperatures at which such semiconductor material is melted andsolidified. For example, the crucible material is suitable for meltingand solidifying semiconductor material at temperatures of at least about300° C., at least about 1000° C. or even at least about 1580° C. fordurations of at least about 10 hours or even as much as 100 hours ormore.

The thickness of the bottom 10 and sidewalls 14 may vary depending upona number of variables including, for example, the strength of materialat processing temperatures, the method of crucible construction, thesemiconductor material of choice and the furnace and process design.Generally, the thickness of the crucible may be from about 5 mm to about50 mm, from about 10 mm to about 40 mm or from about 15 mm to about 25mm.

Coating Compositions

At least a portion of the inner surface 12 of the sidewalls 14 of thecrucible body 5 described above may be coated with a coating compositionincluding silicon nitride and one or more additives and of a sufficientthickness to reduce the amount of oxygen in an ingot subsequently formedin the crucible and/or to enhance an ingot-release characteristic of thecrucible. The additive may be, for example, one or more of binders,dispersants, sintering aids and a medium, diluent, solvent orcombinations thereof. Ingot-release characteristics include the abilityof the ingot to release the ingot during cooling (i.e., ability of thecrucible not to adhere to the ingot) and to release the ingot withoutcausing ingot cracking. Evidence of ingot adhesion includes, forexample, (1) a failure of the ingot to release from the crucible even atroom temperatures, (2) the amount of ingot cracking upon release and/or(3) the presence and amount of semiconductor material stuck to thecrucible after release of the ingot.

The composition may include at least about 5% by weight silicon nitrideand, in other embodiments, at least about 15% by weight or even at leastabout 30% by weight silicon nitride. In various embodiments, the coatingcomposition may include from about 5% to about 50% by weight siliconnitride, from about 15% to about 50% by weight, from about 10% to about40% by weight or from about 30% to about 40% by weight silicon nitride.In some embodiments, the coating composition includes from about 37.0 to37.7% by weight silicon nitride. For purposes of the present disclosure,percent inclusion of components of the “coating composition” or simply“composition” refer to the material applied to the crucible body and notthe coating itself which forms after additional processing steps (e.g.,vaporization of mediums, heating, or sintering, etc.). Percent inclusionof the components of the “coating” (described below under the sectionentitled “Coated Crucibles”) refer to the solid material covering aportion of the crucible body after all processing steps are performedand that covers the crucible body during preparation of an ingot. Unlessdescribed otherwise, listed percentages are given as the percentage ofthe entire composition or entire coating including the component orcomponents being described.

Without being bound by a particular theory, it is believed that whenused in particulate form, the size of the silicon nitride particles canaffect the rheology of the coating composition and affect the ease ofapplication. In some embodiments, the average nominal diameter of theparticulate silicon nitride may be less than about 100 μm. In otherembodiments, the average nominal diameter of the silicon nitride may beless than about 50 μm, less than about 25 μm or even less than about 10μm. Utilization of smaller particle sizes generally results in coatingcompositions with improved fluidity.

The composition may include a medium in which the silicon nitrideremains substantially in particulate form. Generally, it should beunderstood that one or more of the silicon nitride and additives such asa binder, dispersant, sintering aid and the like may not dissolve,partially dissolve or fully dissolve in the medium and the terms“medium,” “diluent,” and “solvent” may be used interchangeably and arenot meant to limit embodiments of the present disclosure to compositionswherein one or more components do or do not dissolve in the medium.

The medium may include an organic compound or may be aqueous. It shouldbe noted, however, that the presence of water in an aqueous solution mayresult in more oxygen in the cured coating which may also result in anincrease in the oxygen level of a semiconductor ingot grown therein.Thus, while aqueous solutions may be utilized without departing from thepresent disclosure, it may be preferable in some embodiments to use anon-aqueous medium. Preferably the medium vaporizes readily during anydrying steps.

The medium may include a C1 to C10 alcohol and may be isopropyl alcoholor ethanol. In some embodiments, at least about 10% by weight of thetotal composition prior to being applied to the crucible is the medium.

In other embodiments, the composition may include at least about 30%, atleast about 50% or even at least about 70% by weight medium. In variousother embodiments, the composition includes from about 10% to about 80%by weight medium, from about 30% to about 70%, from about 40% to about60% or even from about 45% to about 55% by weight medium. Thecomposition may include from about 47.9% to about 50.6% by weightmedium. The composition may include more than one medium with the totalweight fraction of medium in the composition being as described above.

The composition may include one or more binders which act to bind thecoating and, more particularly, the silicon nitride particles, to thecrucible body and to each other after application. Generally, the binderalters the rheology of the coating composition and maintains thedistribution of particles in the medium throughout application anddrying. In some embodiments, the binder is dissolved in the medium. Insome embodiments, the binder is polyvinyl butyral, such as B-76available from Solutia, Inc. (St. Louis, Mo.). In some embodiments, atleast about 0.5%, at least about 2%, at least about 5% or at least about10% or even at least about 15% by weight of the total composition priorto being applied to the crucible is one or more binders. In variousembodiments, the composition includes from about 0.5% to about 20% byweight binder, from about 0.5% to about 10%, from about 0.5% to about 8%by weight binder or from about 2% to about 8% by weight binder. Thecomposition may include from about 5.4% to about 6.8% by weight binder.In some embodiments, the composition does not include a binder. Thecomposition may include more than one binder with the total weightfraction of binder in the composition being as described above.

The coating composition may include a dispersant. Generally, thedispersant acts to prevent the silicon nitride particulate from settlingprior to application of the coating composition to the crucible body.Suitable dispersants generally do not contribute metallic impurities tothe coating composition and are clean burning and decompose during athermal cycle. The combination of plasticizer (described below) anddispersant may be chosen such that drying of the coating withoutcracking or with minimal cracking is realized. In some embodiments, thedispersant is a methyloxirane polymer such as SOLSPERSE® 20000 (LubrizolCorp., Wickliffe, Ohio). The composition may include at least about0.05% by weight dispersant and, in other embodiments, at least about0.1%, at least about 0.5%, at least about 1% or even at least about 5%by weight dispersants. In various other embodiments, the compositionincludes from about 0.05% to about 10% by weight dispersant, from about0.05% to about 5% or from about 0.5% to about 2.5% by weight dispersant.The composition may include from about 1.6% to about 2.1% by weightdispersant. In some embodiments, the composition does not include adispersant. The composition may include more than one dispersant withthe total amount of dispersant in the composition being as describedabove.

The coating composition may include a plasticizer. Suitable plasticizersgenerally do not contribute metallic impurities to the coatingcomposition and are clean burning and decompose during a thermal cycle.Plasticizers and dispersant may be chosen such that drying of thecoating without cracking or with minimal cracking is realized. Theplasticizer may be, for example, polyethylene glycol. Polyethyleneglycol is commercially available as CARBOWAX® 400 (Dow Chemical Co.,Midland, Mich.). The coating composition may include at least about0.5%, at least about 2%, at least about 5%, at least about 10% or evenat least about 15% by weight plasticizer. In various embodiments, thecomposition includes from about 0.5% to about 20% by weight plasticizer,from about 0.5% to about 10%, from about 0.5% to about 8% or from about2% to about 8% by weight plasticizer. The composition may include fromabout 5.4% to about 6.8% by weight plasticizer. In some embodiments, thecomposition does not include a plasticizer. The composition may includemore than one plasticizer with the total amount of plasticizer in thecomposition being as described above.

The composition may include one or more sintering aids, such as forexample, yttria, silica and/or alumina. The sintering aid may be inparticulate form and may be suspended in one or more mediums. Thesintering aid (and in particular yttria) strengthens the coating onceapplied to the crucible body and generally improves adherence of siliconnitride particles to the crucible body and to each other. The coatingcomposition may include at least about 0.1% by weight sintering aid (orsintering aids when more than one is used), at least about 0.5%, atleast about 1%, at least about 5% or at least about 7.5% by weightsintering aid(s).

The mass ratio of sintering agent to silicon nitride may be at leastabout 1:20 and, in other embodiments, is at least about 1:10, at leastabout 1:5, at least about 2:5, at least about 3:5 at least about 4:5 oreven at least about 1:1. In various other embodiments, the ratio ofsintering agent to silicon nitride is from about 1:20 to about 1:1 orfrom about 1:5 to about 1:2.

In embodiments wherein the coating composition comprises yttria, thecoating composition may contain at least about 0.1% by weight yttriaand, in other embodiments, may contain at least about 0.5% yttria, atleast about 1% yttria, at least about 5% yttria, at least about 7.5%yttria, or even at least about 12% by weight yttria. In various otherembodiments, the coating composition includes from about 0.1% to about40% by weight yttria, from about 1% to about 40% by weight yttria orfrom about 1% to about 20% by weight yttria.

Alternatively or in addition to yttria, the composition may includeother components which may act as sintering aids, such as silica and/oralumina. In some embodiments, the composition includes at least about0.1% by weight silica and, in others, at least about 0.5%, at least 1%or at least about 3% by weight silica. In various embodiments, thecomposition includes from about 0.1% to about 10% by weight silica, from0.1% to about 5% silica or from about 1% to about 5% silica.Alternatively or in addition, the composition may include at least about0.1% by weight alumina. In other embodiments, the composition includesat least about 0.5% by weight alumina, at least about 1% or at leastabout 2% by weight alumina. In various embodiments, the compositionincludes from about 0.1% to about 10% by weight alumina, from 0.1% toabout 4% alumina or from about 1% to about 5% alumina. Generally,yttria, silica, alumina or other oxides are used as a sintering aid toincrease the strength and adherence of the silicon nitride coating.

It should be noted that the amount of medium may be increased incompositions containing relatively higher amounts of oxides and,particularly, that contain relatively higher amounts of silica andalumina. Compositions with relatively higher amounts of medium aregenerally characterized by improved fluidity (see Example 1).

Methods for Coating a Crucible

The coating compositions described in the present disclosure may beapplied to at least a portion of the inner surface of a crucible body bychemical vapor deposition, plasma spraying, brushing, aerosol spraying,pouring or any combination of these. Typically, the application is doneunder a ventilated hood at atmospheric pressures and at a temperaturebelow the flash point of the coating composition. The coatingcomposition may be applied in a single application or multiple times toreach a desired thickness. Once the desired thickness is reached, thecoated crucible may be heated to vaporize the binder, medium, dispersantand the like and to leave behind silicon nitride and any oxide additive(e.g., yttria, silica, alumina and the like) as the coating. Thecrucible may also be sintered to cause densification and strengtheningof the coating. It should be noted that in embodiments wherein multipleapplications are used, the crucible may be dried, heated and/or sinteredafter one or more of the intermediate applications in addition to thefinal application.

Generally, the coating compositions herein described may be appliedalone or in combination to at least a portion of the inner surface ofthe sidewall of the crucible or the entire inner surface of the sidewallof the crucible. If the crucible includes more than one sidewall, thecoating composition may be applied to at least a portion of the innersurface of one or more sidewalls or the entire surface of one or moresidewalls and may be applied to the entire inner surfaces of all thesidewalls.

As shown in FIG. 2, for example, the coating may be applied from thebottom of the crucible to a height, H₁. The distance between the bottomof the crucible and H₁ may be at least about 50% of the height of thesidewall. In other embodiments, the distance between the bottom of thecrucible and H₁ is at least about 70% of the height of the sidewall oreven at least about 85% of the height of the sidewall. In someembodiments, H₁ extends above about a solidification line, S₁, describedbelow (FIG. 3) and, in other embodiments, to about the top of thecrucible.

Generally, the volume of semiconductor materials and the proportionalheight of the material within the crucible may vary as the semiconductormaterial solidifies in the crucible. For instance, silicon increases involume as it solidifies. The “solidification line” of the inner surfaceof the crucible extends the circumference of the inner surface of thesidewall (or sidewalls if the crucible has more than one sidewall) andcorresponds to the expected portion of the crucible adjacent the top ofa solidified ingot or the expected elevation of the top of the ingot.The distance between the bottom of the crucible and the solidificationline corresponds to the distance between the bottom of the crucible andthe top of an ingot solidified within the crucible and, when the bottomof the crucible is not uniform in dimension (such as, for example, aconcave-bottomed crucible), the distance from the bottom of the crucibleat a point where it meets the sidewall to the top of the solidifiedingot.

The bottom 10, top, T, and solidification line, S₁, are generallyillustrated in FIG. 3. Generally, the distance between thesolidification line, S₁, and the top, T, of the crucible sidewall isless than about 25% of the height of the sidewall (i.e., about 25% ofthe distance between where the sidewall meets the bottom of the crucibleand the top of the sidewall) in order to maximize the size of thesolidified ingot. In other embodiments, this distance is less than about15% of the sidewall height, less than about 10% or even less than about5% of the height. In some embodiments, this distance is from 0.5% toabout 25% of the sidewall height.

The inner surface of the crucible may be coated with coatingcompositions of embodiments of the present disclosure across the innersurface of the bottom of the crucible and the inner surface of thesidewall from the bottom of the crucible to at least the solidificationline and, in other embodiments, to a distance above the solidificationline. The distance above the solidification line to which the innersidewall is coated may be at least about 0.5% of the height of thesidewall, at least about 1% of the height of the sidewall or even atleast about 3% of the height of the sidewall.

According to some embodiments of the present disclosure, a first coatingcomposition is applied to a first area of the inner surface of thecrucible sidewall and a second coating composition is applied to asecond area of the inner surface of the sidewall. For instance, twocoating compositions may be applied to different portions of thecrucible sidewall with one coating composition being applied to theportions of the sidewall to which the ingot is relatively more likely toadhere and the other coating composition being applied to the otherareas of the sidewall. The second coating composition may contain anadditive that it not present in the first coating composition. Accordingto some embodiments, the first coating includes a medium, siliconnitride, dispersant and a binder. In addition or alternatively, thesecond coating composition may include a medium, silicon nitride, and anoxide additive such as alumina, silica and yttria and/or other oxides(e.g., lanthanides). The silicon nitride and/or oxide additive may besuspended in the medium.

In various embodiments and as shown in FIG. 4, the first composition maybe applied to the inner surface of the sidewall from the bottom 10 ofthe crucible 5 to a height H₁ and the second coating composition may beapplied to the inner surface of the crucible from about H₁ to a heightH₂. The distance between the bottom of the crucible and H₁ may be atleast about 50% of the height of the sidewall and, in other embodiments,is at least about 70% or even at least about 85% of the height of thesidewall. The distance between the bottom of the crucible and the heightH₂ to which the second coating composition may be applied may be atleast about 60% of the height of the sidewall (for instance when H₁ isless than about 60% of the height of the sidewall), at least about 75%(for instance when H₁ is less than about 75% of the height of thesidewall) or even at least about 90% of the height of the sidewall (forinstance when H₁ is less than about 90% of the height of the sidewall).In other embodiments, H₂ extends above the solidification line, S₁, oreven extends to about the top, T, of the crucible (FIG. 3).

In some embodiments, the first coating composition and the secondcoating composition may be applied to overlapping portions of thecrucible sidewall. For instance, the first coating may be applied fromthe bottom 10 of the crucible 5 to a height H₁ and the second coatingcomposition may be applied to a height below H₁ to a height H₂ with thedistance between the bottom of the crucible and H₁ and H₂ being asdescribed previously. In these embodiments, H₁ may even extend to orabove the solidification line, S₁, or even extend to about the top, T,of the crucible.

In embodiments where the second coating composition contains yttria orother oxides and the first coating composition does not contain suchadditives, by limiting the amount of silicon-wetted surface area towhich the second composition is applied, the total oxygen content of thecoating may be reduced with a proportional reduction in the amount ofoxygen in the solidified ingot. Generally, the first composition which,in some embodiments, does not contain oxide additives (e.g., yttria,silica and/or alumina and the like) may be applied to the bottom of thecrucible.

Once a coating composition is applied to a portion of the inner surfaceof the crucible, the composition may be dried to vaporize the medium.Generally, the crucible may be dried under any atmosphere including, forexample, ambient air, nitrogen, argon or mixtures thereof. Generallywhen ambient air is utilized, a substantial portion if not all of themedium vaporizes after about 20 minutes and, in other embodiments, afterabout 30 minutes or even after 40 minutes. By increasing ventilation(e.g., by the use of circulating air), drying times may beproportionally decreased. Generally, the coating is dried when thecoating does not adhere or transfer material to human fingers uponcontact with the coating.

The coating composition may be applied and dried several times toincrease the thickness of the coating. Each application may be air-driedto remove the medium before application of another coat. Alternativelyor in addition, heating may be utilized as described in more detailbelow. In some embodiments, the coating composition is applied and driedat least about 2 times and in other embodiments, at least about 4 timesor even about 8 times. In various embodiments, the coating compositionis applied until a thickness of at least about 50 μm is achieved and, inother embodiments, at least about 100 μm, at least about 250 μm, atleast about 500 μm or even at least about 750 μm. In severalembodiments, the coating composition is applied until a thickness offrom about 50 μm to about 1000 μm, from about 100 μm to about 750 μm orfrom about 250 μm to about 750 μm is achieved. Generally, increasedthickness of the coating result in decreased oxygen content in resultingingots, particularly, if the coating composition itself contains minimalamounts of oxygen.

After the desired thickness is achieved (or thicknesses if more than onerange of thickness is desired on the inner surface of the crucible), thecrucible may be heated to a temperature sufficient to remove any binder,dispersant, plasticizer and the like. Generally, heating may be achievedby heating the crucible with the applied coating composition until thedesired temperature is achieved. The binder, dispersant and/orplasticizer may be removed until the coating contains less than 5% byweight residual carbon or less than about 3% or less than about 1% byweight residual carbon. In some embodiments, the crucible is heated toat least about 150° C., to at least about 200° C., to at least about300° C., to at least about 400° C. or even to at least about 750° C. toremove any binder, plasticizer and dispersant compounds. In variousother embodiments, the crucible may be heated to from about 100° C. to750° C. or from 400° C. to 750° C. The crucible may be heated for atleast about 1 hour and, in other embodiments, at least about 2 hours, atleast about 3 hours or from about 1 hour to about 5 hours. In someembodiments, the crucible is heated to at least about 300° C. for atleast about 2 hours. The atmosphere may be at a pressure (vacuum) offrom about 60 torr (0.08 atm) to about 1 atmosphere or from about 150torr (0.20 atm) to about 1 atmosphere. In other embodiments, pressuresabove atmospheric are used such as, for example, pressures of at least 1atmosphere, at least 2 atmospheres or even at least 5 atmospheres.

The crucible may be heated in the presence of an inert gas such as, forexample, nitrogen, helium or argon. Ambient air may also be used as theatmosphere during heating but is less preferred as it may introduceoxygen into the coating. Further, the materials of construction of thefurnace as well as the gas flows may be controlled to avoid oxidation ofthe coating as appreciated by those of skill in the art.

In addition or alternatively to the crucible heating, the crucible mayalso be sintered to densify the coating. In embodiments wherein thecrucible is heated to remove any residual medium and the binders andplasticizers as described above, sintering may be performed after theremoval of any residual medium, solvent, binder and/or plasticizer orthe sinter may additionally be used to remove one or more of thesecomponents. In certain embodiments, sintering is performed duringmeltdown of a silicon charge to minimize processing time. To achievesintering, the crucible may be heated to a temperature of at least about1000° C. and, in another embodiment, at least about 1100° C. Thecrucible may be sintered for at least about 1 hour and, in otherembodiments, at least about 2 hours, at least about 3 hours or fromabout 1 hour to about 5 hours. In some embodiments, the crucible issintered at a temperature of at least about 1100° C. for at least about3 hours. The crucible may be sintered in the presence of an inert gassuch as, for example, nitrogen, helium or argon. The atmosphere may beat a pressure (vacuum) of from about 60 torr (0.08 atm) to about 1atmosphere or from about 150 torr (0.20 atm) to about 1 atmosphere. Inother embodiments, pressures above atmospheric are used such as, forexample, pressures of at least 1 atmosphere, at least 2 atmospheres oreven at least 5 atmospheres. The residual carbon levels left on thecrucible after different heating and/or sintering treatments were testedin Example 6 below.

When one or more coating compositions are applied to the inner surfaceof the crucible sidewall, the second composition which may include oxideadditives (yttria, silica, alumina and the like) may be applied to thesecond area of the crucible and dried. This may be repeated until thedesired thickness is achieved. The first composition that does notcontain additives may then be applied to the first area of the crucibleand dried. The first composition may be applied and dried repeatedly toachieve the desired thickness. The first coating and second coatingafter applied and/or after dried may overlap each other over a portionof the inner surface of the crucible without departing from the scope ofthe present disclosure.

Coated Crucibles

As discussed above, the coating composition is applied to a portion orall of the inner surface using one or more applications and the coatingcomposition may be dried in various methods depending on the type andnumber of coatings applied and the desired composition of the resultingcoating. In general, by drying the crucible at ambient temperatures themedium vaporizes to leave behind the silicon nitride and a portion ofthe binder, plasticizer, dispersant and/or oxide additives (yttria,silica, alumina and the like). By heating the crucible, the residualbinder, plasticizer and/or dispersant may be vaporized leaving behindsilicon nitride, any oxide additives and residual carbon. As shown inExample 6, by heating the crucible to 400° C. for at least 2 hours, theresidual carbon concentration in the coating may be reduced to less than1% by weight of the coating.

In some embodiments including embodiments wherein the coatingcomposition used to coat a portion of the crucible does not contain anoxide additive, the crucible coating may include at least about 90% byweight silicon nitride. In other embodiments, the coating includes atleast about 95% by weight silicon nitride or even at least about 97.5%by weight silicon nitride. The coating may include less than about 1% byweight carbon. It should be noted that the coating compositionsdescribed in this section entitled “Coated Crucibles” refer tocompositions of the coating after a heating step (e.g., heating to atleast about 300° C.) and, optionally, a sintering step as describedabove in the section above entitled “Methods for Coating a Crucible.”

In various embodiments, the coating extends over the entire innersurface of the crucible sidewall (and in some embodiments sidewalls) ora portion of the inner surface of the sidewall. As described above inrelation to the area to which coating compositions may be applied and asshown in FIG. 2, the coating may extend across the inner surface of thebottom of the crucible and the inner surface of the sidewall from thebottom of the crucible to a height, H₁.

In some embodiments, the coating that covers a first area of thecrucible sidewall may include silicon nitride and an amount of asintering agent (e.g., an oxide additive such as yttria, silica oralumina). In such embodiments, the coating may include at least about40% by weight silicon nitride, at least about 60% silicon nitride oreven at least about 80% by weight silicon nitride in addition tosintering agent.

The coating may include at least about 0.5% by weight yttria. In otherembodiments, the coating may include at least about 1% by weight yttria,at least about 5%, at least about 10%, at least about 15% or even atleast about 20% by weight yttria. In various embodiments, the coatingincludes from about 0.5% to about 25% by weight yttria, from about 0.5%to about 20% by weight, from about 0.5% to about 10% by weight or fromabout 1% to about 20% by weight yttria. The mass ratio of yttria tosilicon nitride in the coating may be at least about 1:20 and, in otherembodiments, at least about 1:10, at least about 2:5 or even at leastabout 4:5.

Alternatively or in addition, oxide additives other than yttria such assilica and/or alumina may be present in the coating. Other suitableoxide additives may include lanthanide oxides. In some embodiments thecoating comprises at least about 0.5% by weight silica. In otherembodiments, the coating may include at least about 1% by weight silica,at least about 5%, at least about 10%, at least about 15%, or even atleast about 20% by weight silica. In various other embodiments, thecoating comprises from about 0.5% to about 25% by weight silica, fromabout 0.5% to about 20% by weight, from about 0.5% to about 10% byweight or from about 1% to about 20% by weight silica.

The crucible coating may include an amount of alumina and, in someembodiments, contains at least about 0.5% by weight alumina. In otherembodiments, the coating includes at least about 1% by weight alumina,at least about 5% or even at least about 10% by weight alumina. Invarious other embodiments, the coating comprises from about 0.5% toabout 25% by weight alumina, from about 0.5% to about 20% by weight,from about 0.5% to about 10% by weight or from about 1% to about 20% byweight alumina.

In particular embodiments, the coating on the inner surface of thecrucible sidewall may consist essentially of silicon nitride, sinteringagent (e.g., yttria, silica or alumina) and carbon. The amount of carbonin the coating may be less than about 1% by weight carbon.

Coated crucibles of the present disclosure may include more than onecoating applied to the inner surface of the crucible sidewall. Thecrucible may include a first coating on a first area of the innersurface of the sidewall and a second coating on a second area of theinner surface of the sidewall. In some embodiments, the second coatingcomprises an additive not present in the first coating such as, forexample, yttria. Alternatively or in addition, the first coating mayinclude an additive not present in the second coating.

Coatings containing sintering agent (e.g., yttria, silica or alumina)may be applied to the entire inner surface of the crucible sidewall ormay be applied to a portion of the sidewall (for instance between H₁ andH₂ as shown in FIG. 4). Such coatings provide improved ingot-releasecharacteristics so it may be desirable to apply the coatings to theportion of the crucible where improved ingot-release characteristics aredesirable, typically the portion near the top of the sidewall in upwarddirectional solidification applications. However, it should also benoted that such coatings may introduce oxygen into the ingot. As such,it may also be desirable to minimize the use of such coatings. Theamount and location of such coating may be determined based on thedesired release characteristics and ingot oxygen content.

As described above in relation to application of the first and secondcoating compositions and with reference to FIG. 4, the first coating mayextend from the bottom of the crucible to a height H₁ and the secondcoating may extend from about H₁ to a height H₂. When the coatedcrucible contains two coatings, the first coating may include siliconnitride and may include residual carbon left from any binder,plasticizer, dispersant and the like. The first coating may include lessthan about 1% by weight carbon. The first coating may even consistessentially of silicon nitride and carbon.

The second coating may include silicon nitride and sintering agent(e.g., yttria, silica or alumina). The second coating may include atleast about 40% by weight silicon nitride, at least about 60% or atleast about 80% by weight silicon nitride. The mass ratio of sinteringagent to silicon nitride in the second coating may be at least about1:20, at least about 1:10, at least about 2:5, or even at least about4:5. The second coating may include at least about 0.5% by weight yttriaor at least about 1%, at least about 5%, at least about 10%, at leastabout 15% or even at least about 20% by weight yttria. In someembodiments, the second coating includes from about 0.5% to about 25% byweight yttria.

The second coating may include an amount of silica and, in someembodiments, includes at least about 0.5% by weight silica, at leastabout 1%, at least about 5%, at least about 10%, at least about 15%, atleast about 20% or from about 0.5% to about 25% by weight silica. Thesecond coating may also include an amount of alumina and, in someembodiments, includes at least about 0.5% by weight alumina, at leastabout 1%, at least about 5%, at least about 10% or from about 0.5% toabout 25% by weight alumina.

The second coating may include silicon nitride and any combination ofsintering agents (e.g., any combination of yttria, silica and alumina).The second coating may consist essentially of silicon nitride, sinteringagent and carbon.

In embodiments where only one coating is applied to the inner surface ofthe crucible sidewall or even when multiple coatings are applied, thethickness of any or even all of the coatings if more than one maycorrespond to the thicknesses described above.

Methods for Preparing an Ingot

One aspect of the disclosure relates to preparation of silicon ingotsand, particularly, to preparation of silicon ingots. In embodimentswhere multicrystalline silicon ingots produced by a directionalsolidification process are desired, polycrystalline silicon may beloaded into a coated crucible to form a silicon charge. Coated cruciblesto which polycrystalline silicon may be applied are generally describedabove. Methods for crystallizing are generally described by K. Fujiwaraet al. in Directional Growth Medium to Obtain High QualityPolycrystalline Silicon from its Melt, Journal of Crystal Growth 292, p.282-285 (2006), which is incorporated herein by reference for allrelevant and consistent purposes.

Once loaded into the coated crucible of the present disclosure, thesilicon charge may be heated to a temperature above about the meltingtemperature of the charge to form a silicon melt. The silicon charge maybe heated to at least about 1410° C. to form the silicon melt and, inanother embodiment, at least about 1450° C. to form the silicon melt.Once the silicon melt has been prepared, the melt may be solidified suchas, for example, in a directional solidification process. The ingot maythen be cut into one or more pieces with dimensions matching several ofthe dimensions of a desired solar cell. Wafers may be prepared byslicing these pieces by, for example, use of a wiresaw to produce slicedwafers.

The multicrystalline silicon produced by directional solidification isan agglomeration of crystal grains with the orientation of the grainsrelative to each other being random due to the high density ofheterogeneous nucleation sites at the crucible wall. The resultingmulticrystalline silicon ingot may have an average nominal crystal grainsize of from about 1 mm to about 15 mm and, in other embodiments, has anaverage nominal crystal grain size of from about 5 mm to about 25 mm orfrom about 5 mm to about 15 mm.

Silicon wafers may be produced by slicing the ingot using, for example,a wiresaw. The resulting silicon wafers have average nominal crystalgrain sizes as described above for multicrystalline ingots.

Ingots with Low Oxygen Content

Generally, it has been found that by utilizing a crucible with a coatingof sufficient thickness that contains minimal sources of oxygen (i.e.,yttria, silica, alumina and the like) and by utilizing C1 to C10 alcoholmediums in coating compositions rather than water and, optionally, bysintering these coatings in an inert atmosphere, resultingmulticrystalline ingots and wafers are characterized by a reduced oxygencontent and proportionally reduced light induced degradation.

Referring now to FIG. 5, ingots prepared according to the presentdisclosure, that is by utilizing a silicon nitride coating in an alcoholmedium with a binder and dispersant, contained less oxygen across allfractions of the ingot (i.e., from top to bottom).

Silicon ingots according to embodiments of the present disclosure mayhave a bottom, a top and a height, H₃, wherein H₃ corresponds to theaverage distance between the bottom and top of the ingot. As shown inFIG. 5, the oxygen concentration in the as grown ingot may generallydecrease from the bottom of the ingot to the top of the ingot, whereinthe top and bottom of the ingot correspond to the top and bottom asgrown in the crucible. It should be noted in this regard that the oxygenconcentration profile as described herein refers to the concentrationprofile of an as grown multicrystalline ingot. In this regard, the ingotmay be free of cut marks exhibited by ingots that have been cut by awiresaw or otherwise. The bottom and top of the ingots as describedherein correspond to the bottom and top of the ingots aftersolidification. This allows the ingots to be characterized by an oxygenconcentration along the height of the ingot as solidified.

The silicon ingot may be produced by a directional solidificationprocess as described above. The ingot may contain multicrystallinesilicon with the average nominal crystal grain size being from about 1mm to about 15 mm. In other embodiments, the average nominal crystalgrain size is from about 5 mm to about 25 mm or from about 5 mm to about15 mm.

In some embodiments, the oxygen concentration of the ingot at a heightabout 20% of H₃ is less than about 4.5 ppma. In other embodiments, theoxygen concentration of the ingot at a height about 20% of H₃ is lessthan about 4.0 ppma, 3.0 ppma or even 2.0 ppma. The oxygen concentrationof the ingot between a height about 20% of H₃ to a height about 80% ofH₃ may be less than about 3.0 ppma and, in other embodiments, is lessthan about 2.0 ppma. In one particular embodiment, the oxygenconcentration of the ingot between the bottom and the top is less thanabout 2.5 ppma and, in another embodiment, less than about 2.0 ppma.

Generally, oxygen content may be measured using Fourier TransformInfrared Spectophotometry (FTIR). For example, the oxygen concentrationmay be measured by cutting 2 mm thick slugs of silicon from the ingot atthe height of interest and measuring the oxygen content by FTIR.

Generally, the shape of the ingot corresponds to the crucible in whichit was solidified and, in some embodiments, the ingot is rectangular oreven square in shape.

Wafers with Low Oxygen Content

As can be seen from FIG. 5, the oxygen concentrations along the entireheight of the ingot of the present disclosure are less than the lowestoxygen concentration exhibited by the conventional ingot. Accordingly,wafers produced along the entire height of the ingot will exhibit alower oxygen content that all wafers produced from the conventionalingot. In some embodiments of the present disclosure, a silicon waferhas an oxygen concentration of less than about 2.5 ppma. In otherembodiments, the silicon wafer has an oxygen concentration of less thanabout 2.25 ppma, of less than about 2 ppma, of less than about 1.75ppma, of less than about 1.5 ppma or even less than about 1.25 ppma. Invarious other embodiments, the wafer has an oxygen content of from about0.1 to about 3 ppma, from about 0.5 to about 3 ppma, from about 0.75 toabout 3 ppma, from about 1 to about 3 ppma, from about 0.75 to about 2.5ppma, from about 0.75 to about 2.25 ppma, from about 0.75 to about 2ppma or from about 0.75 to about 1.75 ppma.

The wafer may contain multicrystalline silicon. The average nominalcrystal grain size may be from about 1 mm to about 15 mm, from about 5mm to about 25 mm or from about 5 mm to about 15 mm. In someembodiments, the wafer is rectangular (including square wafers). Thewafer may be characterized by any shape without departing from the scopeof the present disclosure.

EXAMPLES Example 1 Coating a Crucible with a Silicon Nitride CoatingComposition

The coating composition of Table 1 below was prepared by weighing outthe amount of isopropyl alcohol in a sufficiently large beaker. Theamounts of dispersant (SOLSPERSE® 20000), binder (PVB (B-76)),plasticizer (PEG (CARBOWAX® 400)) and silicon nitride powder wereweighed out separately. The beaker and medium were placed onto a hotplate. A high shear impeller type mixer was placed into the fluid. Thebeaker was capped to minimize loss by evaporation. The medium was warmedto a temperature between 60 to 80° C. while stirring. The PVB was addedand stirred until the binder broke down and the fluid became a tackyviscous fluid. After mixing (about 15 minutes) the fluid color achieveda more clear state.

Dispersant and PEG were added to the stirred fluid while stirring. Thehot plate was turned off and the mixture was stirred for 5 minutes. Thecontents were poured while warm into a polyethylene container. Siliconnitride milling balls (10 mm in diameter) were added to the containerfor milling. A polyethylene lid was screwed onto the open end of thecontainer to minimize leakage of the mixture or evaporation of themedium.

The milling container was opened and the pre-weighed silicon nitridepowder was added. The container was closed and placed onto a millingapparatus that turned at a speed of 60 rpm. The mixture was milled for 6hours to ensure thorough mixing.

The composition was then used to coat the inner surface of a silicacrucible (68 cm×68 cm×42 cm). The slip was brushed onto the surfaceusing a foam applicator common to paint application. The composition wasfirst applied in the corners and then applied to the vertical walls andthen the bottom of the crucible. Any puddling or dripping was brushedout before appreciable medium evaporated and skinned the surface of thedrip. The composition was allowed to dry for about 30 to 45 minutes inair at room temperature in a ventilated area. The composition wasapplied three more times with drying in-between to build up a four-passcoating.

The binder, dispersant and plasticizer were removed by heating to atemperature between 300 to 400° C. for two hours. The crucible was thenheated to a temperature of 1100° C. for three hours to sinter thecoating. The thickness of the coating was 400 μm before sintering andfrom 290 μm to 325 μm after sintering. A silicon charge of 270 kg wasadded to the coated crucible and directionally solidified. Thesolidified ingot released well from the crucible.

TABLE 1 Percentage Inclusion by Weight of Components of the CoatingComposition of Example 1 Component Percentage Inclusion (wt %) SiliconNitride 37.5 Dispersant (SOLSPERSE ® 20000) 1.8 Medium (isopropylalcohol) 49.5 Binder (PVB (B-76)) 5.6 Plasticizer (PEG (CARBOWAX ® 400))5.6

Example 2 Coating a Crucible with a Silicon Nitride Coating Compositionthat Contains Yttria

Compositions 1-3 shown in Table 2 were prepared with increasing amountsof yttria. The portions of the desired mixture were weighed out takinginto account the desired silicon nitride to additive mass ratio. Thesilicon nitride and yttria were mixed and the medium was added followedby additions of the dispersant, PVB and PEG. The mixture wasmechanically mixed for five minutes.

TABLE 2 Percentage Inclusion by Weight of Components of the CoatingCompositions of Example 2 Composition 1 Composition 2 Composition 3Component (wt %) (wt %) (wt %) Silicon Nitride 36.14 34.88 32.61Dispersant 1.73 1.67 1.57 (SOLSPERSE ® 20000) Medium (isopropyl 47.7146.05 43.04 alcohol) Binder (PVB (B-76)) 5.40 5.21 4.87 Plasticizer (PEG5.40 5.21 4.87 (CARBOWAX ® 400)) Yttria 3.61 6.98 13.04

The compositions were then used to coat the inner surface of severalcrucibles. Each composition was brushed onto a silica crucible toachieve a 400 μm thickness. The coated-crucible was heated to 400° C.for three hours and was then heated to 1100° C. for four hours. A 60torr argon gas atmosphere was used during heating. Not taking residualcarbon into account, the coating contained silicon nitride and yttria inthe amounts shown in Table 3.

TABLE 3 Percentage Inclusion by Weight of Components of the Coating ofExample 2 Composition 1 Composition 2 Composition 3 Component (wt. %)(wt. %) (wt. %) Silicon 90.9 83.3 71.4 Nitride Yttria 9.1 16.7 28.6

Increasing the concentration of yttria improved bonding of the coatingto the crucible. The coating produced from Composition 3 was found toadhere best to the crucible.

Example 3 Adjustment of the Amount of Medium to Maintain Fluidity

Compositions A-D shown in Table 4 were prepared with increasing amountsof silica, alumina and yttria. The compositions were prepared andapplied to a crucible as described in Example 2. The amount ofdispersant was increased in each composition until the composition wascharacterized by sufficient fluidity.

TABLE 4 Percentage Inclusion by Weight of Components of the CoatingCompositions of Example 3 Com- Composition Composition Compositionposition Component A (%) B (%) C (%) D (%) Silicon Nitride 34.04 22.4218.01 14.53 Dispersant 1.63 1.08 0.86 0.70 (SOLSPERSE ® 20000) Medium(isopropyl 50.30 59.63 63.49 67.25 alcohol) Binder (PVB (B- 5.08 3.352.69 2.17 76)) Plasticizer (PEG 5.08 3.35 2.69 2.17 (CARBOWAX ® 400))Silica 1.37 3.61 4.35 4.68 Alumina 0.77 2.04 2.46 2.65 Yttria 1.72 4.525.45 5.86

The compositions were then used to coat a portion of the inner surfaceof several silica crucibles to a thickness of 100 nm. Thecoated-crucible was heated to 400° C. for three hours and was thenheated to 1100° C. for four hours. A 60 torr argon gas atmosphere wasused during heating. Not taking residual carbon into account, thecoating contained silicon nitride, yttria, silica and alumina in theweight percentages shown in Table 5.

TABLE 5 Percentage Inclusion by Weight of Components of the Coating ofExample 3 Com- Com- Composition A Composition B position C position DComponent (wt %) (wt %) (wt %) (wt %) Silicon 89.8 68.8 59.5 52.4Nitride Silica 3.6 11.1 14.4 16.9 Alumina 2.0 6.3 8.1 9.6 Yttria 4.513.9 18.0 21.1

Only the top 10% of the inner surface of the crucibles was coated withthe yttria-containing composition. The remainder of the inner surface ofeach crucible was coated with the coating composition of Table 1.

Increasing the amounts of yttria, silica and alumina in the coatingimproved bonding of the coating to the crucible. The coating producedfrom Composition D was found to adhere best to the crucible.

Example 4 Preparation of a Coating Composition without Oxide Additivesand a Coating Composition that contains an Oxide Additive (Silica)

The compositions shown in Table 6 were used as starting compositionsthat may be adjusted to provide the proper fluidity so as to avoiddefects in the coating (e.g., to assure continuity of the coating on thecrucible and to avoid pinholes). The first coating composition did notcontain any oxide additives and is suitable for use on the lower potionsof the crucible. The second coating composition contained silica as anadditive and is suitable for use on upper portions of the crucible.

TABLE 6 Percentage Inclusion by Weight of Components of the CoatingCompositions of Example 4 Component First Composition (%) SecondComposition (%) Silicon Nitride 21.8 20.9 Dispersant 4.3 4.1(SOLSPERSE ® 20000) Medium 64.2 61.6 (isopropyl alcohol) Binder (PVB 3.33.1 (Butvar ®)) Plasticizer (PEG 6.4 6.2 (CARBOWAX ® 400)) Silica — 4.2

Example 5 Comparison of the Oxygen Content of Commercially AvailableIngots and Ingots Prepared Using Coated Crucibles of the PresentDisclosure

The oxygen content of a conventional multicrystalline ingot was comparedagainst multicrystalline ingots solidified in a silica crucible coatedwith the composition of Table 1 and dried and sintered in an argonatmosphere and according to the third to last row of Table 7 (below).The coating was 400 μm thick. 2 mm thick samples of material cuthorizontally from the bricks were taken at various solidified fractions(heights) of the ingot. Data points are interpolated by straight linesin the plots to generate the oxygen concentrations at various pointsalong the height of the crucible as shown in FIG. 5.

Example 6 Determination of the Carbon Content of the Coating DuringDrying

The coating composition of Table 1 was applied to several silicacrucibles and different heat treatments were applied. The results areshown in Table 7 below. Heat treatment temperatures (to removeplasticizer, binder, dispersant and the like) were achieved over a twohour ramp. Sintering temperatures were achieved over a 2 hour ramp fromthe burn-off temperature.

TABLE 7 Residual Carbon within a Crucible Coating after Different HeatTreatments Heat Sinter Residual Heat Treatment Temp. Carbon Air Dried?Treatment? Temp. (° C.) Sinter? (° C.) (wt %) Yes No — No — 14.00 YesYes 180 No — 12.30 Yes Yes 210 No — 11.00 Yes Yes 400 No — 0.57 Yes Yes210 Yes 1100 0.16 Yes Yes 300 Yes 1100 0.20 Yes Yes 400 Yes 1100 0.24

As can be seen from Table 7, a heat treatment at 400° C. is generallysufficient to remove most carbon from the crucible coating.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

1. A composition for coating the inner surface of a crucible to improvean ingot-release characteristic of the crucible, the compositioncomprising a medium, silicon nitride, binder and a sintering agent.
 2. Acomposition as set forth in claim 1 wherein the silicon nitride andsintering agent are particulates that are suspended in the medium.
 3. Acomposition as set forth in claim 1 wherein the medium is a C1 to C10alcohol.
 4. A composition as set forth in claim 1 wherein the medium isisopropyl alcohol.
 5. A composition as set forth in claim 1 wherein thebinder is polyvinyl butyral.
 8. A composition as set forth in claim 1wherein the coating composition comprises at least about 0.1% by weightyttria as a sintering agent.
 9. A composition as set forth in claim 1wherein the mass ratio of sintering agent to silicon nitride in thecoating composition is at least about 1:20.
 10. A composition as setforth in claim 1 wherein the coating composition comprises at leastabout 0.1% by weight silica as a sintering agent.
 11. A composition asset forth in claim 1 wherein the coating composition comprises at leastabout 0.1% by weight alumina as a sintering agent.
 12. A composition asset forth in claim 1 wherein the coating composition comprises at leastabout 5% by weight silicon nitride.
 13. A composition as set forth inclaim 1 wherein the coating composition comprises at least about 15% byweight silicon nitride.
 14. A composition as set forth in claim 1wherein the coating composition comprises at least about 30% by weightsilicon nitride.
 15. A composition as set forth in claim 1 wherein thecoating composition comprises at least about 30% by weight medium.
 16. Acomposition as set forth in claim 1 wherein the coating compositioncomprises at least about 0.5% by weight binder.
 17. A composition as setforth in claim 1 wherein the coating composition comprises at leastabout 5% by weight binder.
 18. A composition as set forth in claim 1wherein the coating composition includes at least about 0.5% by weightdispersant.
 19. A composition as set forth in claim 1 wherein thecoating composition includes at least about 1% by weight dispersant. 20.A composition as set forth in claim 1 wherein the coating compositioncomprises at least about 0.5% by weight plasticizer.